Work order rescheduled out with exception resource shortage even though resource is available

Summary:

We noticed a number of Work orders rescheduled out with its associated Sales order demand under the planning exception "Demand at risk due to Resource shortage". But when checked, the Resource requirement is lesser than the resource availability (Resource requirement = 25 h, Resource availability = 42 h).

Why would planning consider a demand to be at risk due to resource shortage and reschedule out its supply even though enough resource availability exists?

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